Object out of range Excel VBA - vba

Question

So I did a very simple loop VBA to consolidate data from different workbook into single workbook. I got the out of range error keep promting me and I've tried my best to think but it's a dead end for me. Appreciate if can get some input from the seniors.
Sub consolidate()
Application.ScreenUpdating = False
Set mb = ThisWorkbook
myfdr = ThisWorkbook.Path
fname = Dir(myfdr & "\*.csv")
Do Until fname = Empty
If fname <> mb.Name Then
Set wb = Workbooks.Open(myfdr & "\" & fname)
mg = Range("A1").End(xlDown).Row
Range("M4").Value = mg
wb.Worksheets.Copy After:=mb.Sheets(mb.Sheets.Count)
wb.Close SaveChanges:=False
n = n + 1
End If
fname = Dir
Loop
Application.ScreenUpdating = True
MsgBox n & "Done"
End Sub
Sub Union()
Application.ScreenUpdating = False
Set ms = Worksheets("Sheet1")
fsn = 1
k = 0
Set mb = ThisWorkbook
myfdr = ThisWorkbook.Path
fname = Dir(myfdr & "\*.csv")
Do Until fname = Empty
If fname <> mb.Name Then
sn = Mid(fname, 1, Len(fname) - 4)
Set cs = Worksheets(sn) '<<<<The subscript out of range error happened here
If fsn = 1 Then
fsn = 0
For g = 1 To cs.Cells(4, 13)
k = k + 1
For r = 1 To 10
ms.Cells(k, r) = cs.Cells(g, r)
Next r
Next g
Else
For g = 9 To cs.Cells(4, 13)
k = k + 1
For r = 1 To 10
ms.Cells(k, r) = cs.Cells(g, r)
Next r
Next g
End If
End If
fname = Dir
Loop
End Sub